This invention relates to protective devices for birdhouses, and, more particularly, to an entrance which prevents undesirable birds from entering a purple martin house.
Purple martins are beneficial birds and many people maintain birdhouses to attract them. Purple martin houses are generally constructed from metal, plastic or wood. Purple martin houses made from hollow gourds and molded plastic artificial xe2x80x9cgourdsxe2x80x9d are also common. Traditionally, all of these types of purple martin houses have had circular entrance openings.
It has long been recognized that predatory birds, such as starlings, owls and hawks, will enter purple martin houses and either kill or evict the purple martins. European starlings, which were introduced into America during the nineteenth century, present a particularly serious threat to purple martins. Various devices, such as those described in U.S. Pat. Nos. 5,228,410 and 1,166,100, have been used to make the entrance opening of birdhouses smaller in order to prevent entry by larger undesirable birds. While such devices will exclude larger predators, such as owls and hawks, from purple martin houses, they will not exclude starlings, which are substantially the same size as purple martins. Starlings are notorious for attempting to take over martin nests by killing or injuring the martins, and by despoiling the nests. Also, nesting starlings will chase off investigating martins, ruining the chance of establishing a colony site. Traps, such as those described in U.S. Pat. Nos. 5,172,651 and 4,471,4721, may discourage starlings, but will not positively exclude them from purple martin houses.
It has recently been found that purple martins will readily use an entrance opening that is in the shape of an oval or a circle segment, but that starlings are somewhat reluctant to enter such openings. Use of such an opening would reduce the likelihood that a starling will enter a purple martin house, but does not completely eliminate this possibility.
This invention provides an entrance for a purple martin house having an opening with two posts projecting vertically from the bottom edge toward the top edge. The spacing between the posts is such that a bird of the size of an adult purple martin and starling cannot pass between them without opening its wings. Sufficient space is provided between the outer edges of the posts and the side edges of the entrance opening to provide clearance for the wings of a bird as it passes between the posts. It has been found that purple martins will readily use such openings, but that starlings will not do so. Starlings will thus be excluded from a birdhouse having such an entrance opening. Because the opening is too small for them, larger predators, such as owls and hawks, will also be excluded.
The spacing between the outer edges of the posts and the side edges of the entrance opening is also made wide enough that a purple martin chick can reach through it for feeding. The entrance opening will thus allow at least three young birds to present themselves for feeding by a parent at the same time.